Heat waves rather than continuous warming exacerbate impacts of nutrient loading and herbicides on aquatic ecosystems

Effects of multiple stressors on freshwater food webs: Evidence from a mesocosm experiment

Natural and anthropogenic pressures exert influence on ecosystem structure and function by affecting the physiology and behavior of organisms, as well as the trophic interactions within assemblages. Therefore, understanding how multiple stressors affect aquatic ecosystems can improve our ability to manage and protect these ecosystems and contribute to understanding fundamental ecological principles. Here, we conducted a mesocosm experiment to ascertain the individual and combined effects of multiple stressors on trophic interactions within species in freshwater ecosystems. Furthermore, we investigated how species respond to such changes by adapting their food resources. To mimic a realistic food web, we selected fish and shrimp as top predators, gastropods, zooplankton and zoobenthos as intermediate consumers, with producers (macrophytes, periphyton and phytoplankton) and detritus as basal resources. Twelve different treatments included a control, nutrient loading only, herbicide exposure only, and a combination of nutrient loading and herbicide exposure, each replicated under ambient temperature, constant warming and multiple heat waves to simulate environmental stressors. Our results demonstrated that antagonistic interactions between environmental stressors were widespread in trophic interactions, with a more pronounced and less intense impact observed for the high trophic level species. The responses of freshwater communities to environmental stressors are complex, involving direct effects on individual species as well as indirect effects through species interactions. Moreover, our results confirmed that the combinations of stressors, but not individual stressors, led to a shift to herbivory in top predators, indicating that multiple stressors can be more detrimental to organisms than individual stressors alone. These findings elucidate how changes in the resource utilization of species induced by environmental stressors can potentially influence species interactions and the structural dynamics of food webs in freshwater ecosystems.

Responses of Protozoan Communities to Multiple Environmental Stresses (Warming, Eutrophication, and Pesticide Pollution)

To explore the impacts of multiple environmental stressors on animal communities in aquatic ecosystems, we selected protozoa-a highly sensitive group of organisms-to assess the effect of environmental change. To conduct this simulation we conducted a three-factor, outdoor, mesocosm experiment from March to November 2021. Changes in the community structure and functional group composition of protozoan communities under the separate and combined effects of these three environmental stressors were investigated by warming and the addition of nitrogen, phosphorus, and pesticides. The results were as follows: (1) Both eutrophication and pesticides had a considerable promotional effect on the abundance and biomass of protozoa; the effect of warming was not considerable. When warming was combined with eutrophication and pesticides, there was a synergistic effect and antagonistic effect, respectively. (2) Eutrophication promoted α diversity of protozoa and affected their species richness and dominant species composition; the combination of warming and pesticides remarkably reduced the α diversity of protozoa. (3) Warming, eutrophication, and pesticides were important factors affecting the functional groups of protozoa. Interaction among different environmental factors could complicate changes in the aquatic ecological environment and its protozoan communities. Indeed, in the context of climate change, it might be more difficult to predict future trends in the protozoan community. Therefore, our results provide a scientific basis for the protection and restoration of shallow lake ecosystems; they also offer valuable insights in predicting changes in shallow lakes.

Response of dissolved carbon dioxide and methane concentration to warming in shallow lakes

Shallow lake ecosystems are highly sensitive to temperature fluctuation because of their high water surface-to-volume ratios. Shallow lakes have been increasingly identified as a hotspot of CO2 and CH4 emissions, but their response to temperature variation remains unclear. Here, we report from a 5-month outdoor mesocosm experiment where we investigated the impacts of a projected 3.5 °C future warming and monthly temperature changes on lake CO2 and CH4, as well as the key drivers affecting the lake carbon cycling. Our results show that CO2 and CH4 concentrations had a significantly positive correlation with monthly temperatures. CH4 concentration was primarily regulated by monthly temperature, while nutrients effects on CO2 concentration overrode climate warming and temporal temperature changes. These findings imply the varied roles that temperature and nutrient levels can play on CO2 and CH4 dynamics in shallow lake systems. The relationship between temperature and CO2 concentration was nonlinear, showing a threshold of approximately 9 °C, at which CO2 concentration could be strongly modified by nutrient level in the lake systems. Understanding this complex relationship between temperature with CO2 and CH4 concentrations in shallow lakes is crucial for effective lake management and efficient control of greenhouse gases (GHGs) emissions.

The combination of multiple environmental stressors strongly alters microbial community assembly in aquatic ecosystems

Microorganisms play a critical role in maintaining the delicate balance of ecosystem services. However, the assembly processes that shape microbial communities are vulnerable to a range of environmental stressors, such as climate change, eutrophication, and the use of herbicides. Despite the importance of these stressors, little is known about their cumulative impacts on microbial community assembly in aquatic ecosystems. To address this knowledge gap, we established 48 mesocosm experiments that simulated shallow lake ecosystems and subjected them to warming (including continuous warming (W) and heat waves (H)), glyphosate-based herbicides (G), and nutrient loading (E). Our study revealed that in the control group, both deterministic and stochastic processes codominated the assembly of microbial communities in water, whereas in sediment, the processes were primarily stochastic. Interestingly, the effects of multiple stress factors on assembly in these two habitats were completely opposite. Specifically, stressors promoted the dominance of stochastic processes in water but increased the importance of deterministic processes in sediment. Furthermore, warming amplified the effects of herbicides but exerted an opposite and stronger influence on assembly compared to nutrients, emphasizing the complexity of these mechanisms and the significance of considering multiple stressors. The interaction of some factors significantly affected assembly (p < 0.05), with the effects of WEG being most pronounced in water. Both water and sediment exhibited homogeneous assembly of microbial communities (mean NTI >0), but the phylogenetic clustering of microbial communities in water was more closely related (NTI >2). Our research revealed the response model of microbial community assembly in aquatic ecosystems to multiple environmental stresses, such as agricultural pollution, climate change, and eutrophication, and indicated that microbial community changes in sediment may be an important predictor of lake ecosystem development. This provides scientific evidence that better environmental management can reduce impacts on aquatic ecosystems under the threat of future warming.

Use of CMIP6 scenarios as a reference to understand the responses of macrophyte germination and seedling growth to future warming and allelopathy co-stressors

The application of appropriate references such as CMIP6 climate scenarios for benchmarking studies of climate change on ecosystems can promote consistency among different climate change research. However, the use of CMIP6 climate scenarios is not common among experiments on the effects of climate change on freshwater ecosystems. Also, little is known about the impact of ecological factor such as allelopathy of alien species on macrophyte germination and seedling growth under different climate scenarios. In our study, by simulating three annual mean temperature changes at global warming levels of 1.5 °C (low warming scenario), 2 °C (medium warming scenario) and 4 °C (high warming scenario) corresponding to CMIP6 multi-model mean change at the corresponding global warming level, we conducted a mesocosm experiment to investigate their possible effects of different climate scenarios and allelopathy co-stressors on macrophyte germination and seedling growth. Our study showed that three warming scenarios all can facilitate macrophyte propagule germination and seedling growth, but the effect paths vary with CMIP6 warming scenarios and there are more influence pathways under high warming scenarios than under low and medium warming scenarios. Higher aqueous extract concentrations of Eichhornia crassipes can significantly stimulate macrophyte propagule germination and seedling growth. And the medium and high warming scenarios may exacerbate the impacts of allelopathic substances on macrophyte germination and seedling growth, and their effects depend on the combination of the two stressors. These results indicated that medium- and high-temperature scenarios may have greater ecological effects on macrophytes than low-temperature scenarios. Thus, our results highlighted that future climate studies need proper benchmarks such as CMIP6 warming scenarios, because it can provide relatively more accurate and realistic simulations, valid comparative results, comprehensive understanding and supportive coordination among researchers.

Effects of nutrient enrichment on freshwater macrophyte and invertebrate abundance: A meta-analysis

External nutrient loading can cause large changes in freshwater ecosystems. Many local field and laboratory experiments have investigated ecological responses to nutrient addition. However, these findings are difficult to generalize, as the responses observed may depend on the local context and the resulting nutrient concentrations in the receiving water bodies. In this research, we combined and analysed data from 131 experimental studies containing 3054 treatment-control abundance ratios to assess the responses of freshwater taxa along a gradient of elevated nutrient concentrations. We carried out a systematic literature search in order to identify studies that report the abundance of invertebrate, macrophyte, and fish taxa in relation to the addition of nitrogen, phosphorus, or both. Next, we established mixed-effect meta-regression models to relate the biotic responses to the concentration gradients of both nutrients. We quantified the responses based on various abundance-based metrics. We found no responses to the mere addition of nutrients, apart from an overall increase of total invertebrate abundance. However, when we considered the gradients of N and P enrichment, we found responses to both nutrients for all abundance metrics. Abundance tended to increase at low levels of N enrichment, yet decreased at the high end of the concentration gradient (1-10 mg/L, depending on the P concentration). Responses to increasing P concentrations were mostly positive. For fish, we found too few data to perform a meaningful analysis. The results of our research highlight the need to consider the level of nutrient enrichment rather than the mere addition of nutrients in order to better understand broad-scale responses of freshwater biota to eutrophication, as a key step to identify effective conservation strategies for freshwater ecosystems.

Unveiling novel perspectives on niche differentiation and plasticity in rhizosphere phosphorus forms of submerged macrophytes with different stoichiometric homeostasis

Stoichiometric homeostasis is the ability of organisms to maintain their element composition through various physiological mechanisms, regardless of changes in nutrient availability. Phosphorus (P) is a critical limiting element for eutrophication. Submerged macrophytes with different stoichiometric homeostasis regulated sediment P pollution by nutrient resorption, but whether and how P homeostasis and resorption in submerged macrophytes changed under variable plant community structure was unclear. Increasing evidence suggests that rhizosphere microbes drive niche overlap and differentiation for different P forms to constitute submerged macrophyte community structure. However, a greater understanding of how this occurs is required. This study examined the process underlying the metabolism of different rhizosphere P forms of submerged macrophytes under different cultivation patterns by analyzing physicochemical data, basic plant traits, microbial communities, and transcriptomics. The results indicate that alkaline phosphatase serves as a key factor in revealing the existence of a link between plant traits (path coefficient = 0.335, p < 0.05) and interactions with rhizosphere microbial communities (average path coefficient = 0.362, p < 0.05). Moreover, this study demonstrates that microbial communities further influence the niche plasticity of P by mediating plant root P metabolism genes (path coefficient = 0.354, p < 0.05) and rhizosphere microbial phosphorus storage (average path coefficient = 0.605, p < 0.01). This research not only contributes to a deeper comprehension of stoichiometric homeostasis and nutrient dynamics but also provides valuable insights into potential strategies for managing and restoring submerged macrophyte-dominated ecosystems in the face of changing nutrient conditions.

Chemodiversity of Cyanobacterial Toxins Driven by Future Scenarios of Climate Warming and Eutrophication

Climate change and eutrophication are two environmental threats that can alter the structure of freshwater ecosystems and their service functions, but we know little about how ecosystem structure and function will evolve in future scenarios of climate warming. Therefore, we created different experimental climate scenarios, including present-day conditions, a 3.0 °C increase in mean temperature, and a "heatwaves" scenario (i.e., an increase in temperature variability) to assess the effects of climate change on phytoplankton communities under simultaneous stress from eutrophication and herbicides. We show that the effects of climate warming, particularly heatwaves, are associated with elevated cyanobacterial abundances and toxin production, driven by a change from mainly nontoxic to toxic Microcystis spp. The reason for higher cyanobacterial toxin concentrations is likely an increase in abundances because under the dual pressures of climate warming and eutrophication individual Microcystis toxin-producing ability decreased. Eutrophication and higher temperatures significantly increased the biomass of Microcystis, leading to an increase in the cyanobacterial toxin concentrations. In contrast, warming alone did not produce higher cyanobacterial abundances or cyanobacterial toxin concentrations likely due to the depletion of the available nutrient pool. Similarly, the herbicide glyphosate alone did not affect abundances of any phytoplankton taxa. In the case of nutrient enrichment, cyanobacterial toxin concentrations were much higher than under warming alone due to a strong boost in biomass of potential cyanobacterial toxin producers. From a broader perspective our study shows that in a future warmer climate, nutrient loading has to be reduced if toxic cyanobacterial dominance is to be controlled.

Microcosm experiment combined with process-based modeling reveals differential response and adaptation of aquatic primary producers to warming and agricultural run-off

Fertilizers, pesticides and global warming are threatening freshwater aquatic ecosystems. Most of these are shallow ponds or slow-flowing streams or ditches dominated by submerged macrophytes, periphyton or phytoplankton. Regime shifts between the dominance of these primary producers can occur along a gradient of nutrient loading, possibly triggered by specific disturbances influencing their competitive interactions. However, phytoplankton dominance is less desirable due to lower biodiversity and poorer ecosystem function and services. In this study, we combined a microcosm experiment with a process-based model to test three hypotheses: 1) agricultural run-off (ARO), consisting of nitrate and a mixture of organic pesticides and copper, differentially affects primary producers and enhances the risk of regime shifts, 2) warming increases the risk of an ARO-induced regime shift to phytoplankton dominance and 3) custom-tailored process-based models support mechanistic understanding of experimental results through scenario comparison. Experimentally exposing primary producers to a gradient of nitrate and pesticides at 22°C and 26°C supported the first two hypotheses. ARO had direct negative effects on macrophytes, while phytoplankton gained from warming and indirect effects of ARO like a reduction in the competitive pressure exerted by other groups. We used the process-based model to test eight different scenarios. The best qualitative fit between modeled and observed responses was reached only when taking community adaptation and organism acclimation into account. Our results highlight the importance of considering such processes when attempting to predict the effects of multiple stressors on natural ecosystems.

Buffering capacity of submerged macrophytes against nutrient pulses increase with its coverage in shallow lakes

Submerged macrophytes can improve water quality and buffer the effects of external nutrient loading, which helps to maintain a clear-water state in shallow lakes. We constructed 12 large enclosures with contrasting coverages (treatments) of submerged macrophytes (SMC) to elucidate their buffering capacity and resilience to nutrient pulses. We found that aquatic ecosystems with high SMC had higher buffering capacity and resilience, vice versa, i. e, the enclosures with high SMC quickly buffered the nutrient pulse and rebounded to clear-water state after a short stay in turbid-water state dominated by algae, while the treatments with low SMC could not fully buffer the pulse and rebound to clear-water state, and they slowly entered the transitional state after staying in turbid-water state. This means that the enclosures with high SMC had a better water quality than those with low SMC, i.e., the levels of nutrients and Chl-a were lower in the treatments with high plant coverage. In addition, plant coverage had a significantly positive buffering effect against nitrogen and phosphorus pulses, i.e., the nutrient concentrations in the treatments with high SMC took shorter time to return to the pre-pulse level. Overall, our results evidenced that the higher that the SMCs is, the better is the water quality and buffering capacity against nutrient pulses, i.e. the more stable is the clear-water state. However, low SMC may not be able to resist the impact of such strong nutrient pulse. Our results provide reference and guidance for water pollution control and water ecological restoration.

Stoichiometric stability of aquatic organisms increases with trophic level under warming and eutrophication

The balance of stoichiometric traits of organisms is crucial for nutrient cycling and energy flow in ecosystems. However, the impacts of different drivers on stoichiometric (carbon, C; nitrogen, N; and phosphorus, P) variations of organisms have not been well addressed. In order to understand how stoichiometric traits vary across trophic levels under different environmental stressors, we performed a mesocosm experiment to explore the impacts of warming (including +3 °C consistent warming above ambient and heat waves ranging from 0 to 6 °C), eutrophication, herbicide and their interactions on stoichiometric traits of organisms at different trophic levels, which was quantified by stable nitrogen isotopes. Results showed that herbicide treatment had no significant impacts on all stochiometric traits, while warming and eutrophication significantly affected the stoichiometric traits of organisms at lower trophic levels. Eutrophication increased nutrient contents and decreased C: nutrient ratios in primary producers, while the response of N:P ratios depended on the taxonomic group. The contribution of temperature treatments to stoichiometric variation was less than that of eutrophication. Heat waves counteracted the impacts of eutrophication, which was different from the effects of continuous warming, indicating that eutrophication impacts on organism stoichiometric traits depended on climate scenarios. Compared to environmental drivers, taxonomic group was the dominant driver that determined the variations of stoichiometric traits. Furthermore, the stoichiometric stability of organisms was strongly positively correlated with their trophic levels. Our results demonstrate that warming and eutrophication might substantially alter the stoichiometric traits of lower trophic levels, thus impairing the nutrient transfer to higher trophic level, which might further change the structure of food webs and functions of the ecosystems.